1. Field of Invention
The present invention provides a method of removing spectator ions from aqueous suspensions of solid particles.
2. Description of Related Art
Barium titanate (BaTiO3, hereinafter sometimes abbreviated as “BT”) is one of the most widely used ceramic materials in the electronics industry, especially in the production of multilayer capacitors. Many techniques have been developed over the years to produce BT, but the most common technique for producing BT on an industrial scale is a wet chemical process known as the Clabaugh process.
In the Clabaugh process, aqueous barium chloride (BaCl2.2H2O) and titanium tetrachloride (TiCl4) solutions are mixed with an aqueous solution of oxalic acid (HOOCCOOH.2H2O) to produce barium titanyl oxalate tetrahydrate (BaTiO(C2O4)2.4H2O, hereinafter sometimes abbreviated as “BTO”). The reaction can be carried out a room temperature and atmospheric pressure. When calcined at 750° C., BTO decomposes to BT.
For many electronic applications, it is critical that the BT be stoichiometric. In other words, the ratio of barium atoms to titanium atoms in the BT must be very close to unity (Ba:Ti=1.0). To produce stoichiometric BT using the Clabaugh process, a 5% molar excess of barium chloride and a 20% molar excess of oxalic acid must be used to minimize competing reactions. The Clabaugh process is entirely aqueous, but the resulting BTO precipitate must be washed in order to remove chlorine and excess barium.
The use of the BTO as a BT precursor has many advantages over other processes to produce BT, such as high temperature solid state reactions between mixtures of oxides and hydrothermal techniques. BTO particle size, stoichiometry and secondary phases can be controlled and the material can be easily produced at room temperature and atmospheric pressure from a variety of relatively inexpensive materials. There are, however, some disadvantages to the wet chemical technique.
Depending on the method used, production of large quantities of BTO can be time consuming. Moreover, if the BTO particles are left in aqueous suspension for extended periods of time, agglomeration and particle growth (Ostwald ripening) can readily occur. If agglomerates are not removed before calcination, they may lead to the formation of hard aggregates that can compromise the packing efficiency of the final BT powder. Furthermore, the wet chemical process requires several washing steps to remove spectator ions, such as chloride, excess barium and/or nitrates, which could harm electronic properties in the final BT.
Recovery and washing of the BTO precipitate is normally the most expensive and time-consuming step in the BT production process. BTO particles may be recovered by centrifugation or filtration, but these techniques bring the BTO particles into close contact with one another, possibly causing irreversible aggregation. This can be prevented with the use of polymeric flocculants that separate particles through a steric mechanism. Depending on the material, these additives may be expensive and may introduce unwanted ions into the material.
One of the most common techniques for the recovery of BTO is displacement washing in which the aqueous BTO suspension is centrifuged, the supernatant decanted, and the BTO powder redispersed with fresh solution. Several repetitions of this washing process are normally required to remove the contaminants to an acceptable level. This is time consuming and difficult to apply to large volumes of material. Also, depending on the wash solution, leaching or dissolution of the recovered material may occur. Thus, care must be taken in order to avoid possible complexation reactions between ions in solution, such as acetate, and ions in the solid, especially Ba2+.